An overall configuration of a conventional radiation imaging apparatus will be described with reference to FIG. 13.
FIG. 13(a) is a general embodiment of the conventional radiation imaging apparatus. The conventional radiation imaging apparatus is configured by a radiation source 1 for irradiating on a subject M, an image receiving means 4 including a two-dimensional radiation detector 2 for detecting transmitted radiation R of the subject M and converting the same to image signals and an anti-scatter grid 3 arranged on a front face of the two-dimensional radiation detector 2, a video system 5 formed by holding the radiation source 1 and the image receiving means 4 so as to face each other with a predetermined spacing, and an image processing device 6 for performing a predetermined correction process and saving/displaying the image signals obtained with the two-dimensional radiation detector 2.
The radiation detector configured as above operates as below. When radiation R′ is applied from the radiation source 1, one part of the radiation R′ transmits through the subject M and reaches the image receiving means 4 (hereinafter referred to as direct radiation Rd′). As shown in FIG. 13(b), the direct radiation Rd′ transmitted through the anti-scatter grid 3 and reached to the two-dimensional radiation detector 2 are hereinafter referred to as transmitted direct radiation Rd. The intensity distribution of the direct radiation Rd′ changes by a spatial distribution of the transmissivity of the subject M, and the intensity distribution of the transmitted direct radiation Rd changes by a spatial distribution of the transmissivity of the anti-scatter grid 3.
One part of the radiation R′ scatters in the subject M and enters the image receiving means 4 through a path different from the direct radiation Rd′ (hereinafter referred to as scattered radiation Rs′). The scattered radiation Rs′ transmitted through the anti-scatter grid 3 and reached to the two-dimensional radiation detector 2 are hereinafter referred to as transmitted scattered radiation Rs.
The total of the transmitted direct radiation Rd and the transmitted scattered radiation Rs, that is, the total radiation reaching the two-dimensional radiation detector 2 is referred to as transmitted radiation R.
The anti-scatter grid 3 has a structure in which a plurality of radiation shielding plates 31 is arranged at equal intervals, and an intermediate substance 32 having transmissivity is filled between the radiation shielding plates 31. The radiation shielding plate 31 is arranged so as to be tilted the more distant from a center part according to the distance between the radiation source 1 and the anti-scatter grid 3. As the anti-scatter grid 3 is configured as above, the direct radiation Rd′ entered to the radiation shielding plate 31 is absorbed, but the direct radiation Rd′ entered to the intermediate substance 32 is transmitted to reach the two-dimensional radiation detector 2. The majority of the scattered radiation Rs′ entered to the intermediate substance 32, on the other hand, is absorbed by the adjacent radiation shielding plate 31 and does not reach the two-dimensional radiation detector 2. Thus, with the ratio that the direct radiation Rd′/scattered radiation Rs′ transmit through the anti-scatter grid 3 being defined as the direct radiation transmissivity/scattered radiation transmissivity respectively the anti-scatter grid 3 has high direct radiation transmissivity and low scattered radiation transmissivity As a result, the majority of the direct radiation Rd′ reaches the two-dimensional radiation detector 2, and the majority of the scattered radiation Rs′ is absorbed by the anti-scatter grid 3 and do not reach the two-dimensional radiation detector 2, and thus lowering in image quality due to the influence of scattered radiation can be lightened.
However, since one part of the direct radiation Rd′ entered into the radiation shielding plate 31 is absorbed, periodic shade (moire) of the radiation shielding plate 31 produces at the two-dimensional radiation detector 2. A method for reducing the moire produced on the image with the pitch of the scattered radiation shielding plate 31 as an integral multiple of the pitch of the pixel column is proposed (e.g., patent document 1). According to this method, the reduction of the periodic image signals generated by the absorption of the scattered radiation shielding plate 31 is corrected based on the image signals taken in a state with no scattered radiation state in advance.
In either case, it is desirable to absorb the scattered radiation Rs′ as much as possible so as not to reach the two-dimensional radiation detector 2 to obtain a clearer image. In other words, it is desirable to have a small pitch for the radiation shielding plate 31, high height h or thick thickness t to enhance the absorptance of the scattered radiation from the standpoint of anti-scattering.
[Patent document 1] Japanese Laid-Open Patent Publication No. 2002-257939